Thermoelectric Properties and Local Electronic Structure of Rare Earth-doped Ca3Co2O6

Thermoelectric properties of a series of rare earth metal-doped polycrystalline samples of (Ca1-xRx)3Co 2O6 with R = Gd, Tb, Dy and Ho (x = 0 - 0.1) were investigated in the temperature range from 300 K to 1300 K. In a high temperature region above 900 K, a partial rare earth substitution with R 3+ for Ca2+ resulted in appreciable increase in the Seebeck coefficient (S). However, the S value decreased abruptly at low temperatures, and turned to negative values for the Gd- and Tb-doped samples at temperatures below 400 K. With decreasing ionic radii of rare earth elements (Gd3+ > Tb3+ > Dy3+ > Ho3+), the S values increased, while the thermal conductivity (kappa) decreased particularly at temperatures above 700 K. Contrastingly, the influence of rare earth metal substitution on the electrical resistivity (rho) was small; in high temperature region the rho values increased only slightly with decreasing ionic radii of rare earth metals. High-temperature thermoelectric figure-of-merit (Z) of the samples was thereby improved by the late rare-earth metal substitution for Ca2+, particularly for those with Ho3+. A maximum Z value of the Ho-doped sample for x = 0.03 was 1.83 times 10-4 K-1 at 1100 K as compared with 0.37 times 10-4 K-1 for non-doped sample. The electronic structure of the samples was also investigated by x-ray photoemission spectroscopy (XPS) technique. The charge-transfer satellite structure of Co 2p core-level spectra was observed for the Gd-and Tb-doped samples, while the satellite is negligible for the other samples.